1. Field of the Invention
This invention relates to an optical head for writing and/or reading out information signals for an optical recording medium, and to a recording and/or reproducing apparatus for recording and/or reproducing information signals for the optical recording medium using the optical head.
2. Description of Related Art
Up to now, there have been provided a large variety of optical recording mediums, including a replay-only optical disc, including information signals, such as audio signals, video signals or programs, prerecorded thereon, a recordable (R) optical discs, capable of writing the information signals only once, such as DVD-R or DVD+R, rewritable (RW) optical discs, capable of repeatedly rewriting the information signals, such as DVD-RW or DVD+RW, and magneto-optical discs. These discs may also each be rotationally accommodated within a disc cartridge.
In a disc substrate of the optical recording medium, which permits signal writing thereon, such as the recordable disc, rewritable disc or the magneto-optical disc, a guide groove(s), known as a (pre) groove(s), is formed concentrically or spirally about the center opening of the disc as the center. A recording track(s) is formed in register with this groove. The area defined between neighboring grooves or neighboring turns of the groove is termed a land, which may also be used as a recording track.
In an optical head for writing and/or reading out information signals for the optical recording medium, including the groove, formed therein, a light beam radiated from a light source is condensed by an objective lens on a recording track, and a tracking operation is performed in order to cause a spot of the light beam, condensed by the objective lens, to follow the recording track, based on a signal obtained from the light beam reflected and diffracted by the recording track.
In certain types of these optical recording mediums, there is formed a wobbling groove for meandering (wobbling) with a preset period, for providing a wobbled recording track which per se carries the recording clocks, address information or the information relevant to the optical recording mediums. That is, in these types of the optical recording mediums, the aforementioned information is recorded as wobble signals, corresponding to the above information, modulated by wobbling, on the recording track. For example, in the optical discs, exemplified by the DVD+RW or the Blue-Ray Disc, the address signals by wobble signals, different in frequency or in phase, are superimposed on mono-tonal clock signals obtained by high frequency wobbling, for recording the address timing information as wobble signals to extremely high precision. These wobble signals may be detected in accordance with a push-pull method, that is, by receiving the light beam, reflected and diffracted by the recording track, by a photodetector device, split in two light receiving sections along a splitting line extending along the recording track, and by taking the difference of outputs from the two light receiving sections of the so split photodetector device.
Meanwhile, the modulated signals, by the aforementioned wobbling, are modulated, in many cases, by MSK (minimum shift keying) or PSK (phase shift keying), which is a sort of the FSK (frequency shift keying), often used in for example the digital signal transmission.
However, in the modulation used for an optical recording medium, as distinct from the modulation used in e.g. the communication, characteristic signal deterioration may be produced in the wobble signals, as detected by the optical head, in case wobbling is applied to both the neighboring recording tracks. Specifically, the variations in the amplitude or the phase, known as beat or crosstalk, are produced in the aforementioned wobble signals due to the interference between the neighboring recording tracks caused by the difference in the wobbling applied to these recording tracks.
The most outstanding example is the phenomenon known as “beat” in which, due to periodic changes in the phase relationship of the wobbling from the difference between the inner and outer rims of two neighboring recording tracks, the wobble amplitude for the out-of-phase state shown in FIG. 23B is increased by a factor of approximately two, from that for the in-phase state shown in FIG. 23A, even in a standard condition substantially free of e.g. the aberration, thus producing periodic amplitude variations in the wobble signals.
Specifically, when the wobbling of a given recording track is in-phase with the wobbling of the recording track neighboring thereto, wobbling proceeds as the distance between the centerlines of two neighboring tracks remains coincident with the track pitch. Thus, the light beam reflected and diffracted by the recording track contains the diffracted light deviated by λ/(Tp·NA) from the main light beam in a direction perpendicular to the recording track. In the above formula, Tp is the track pitch of the optical recording medium, λ is the wavelength of the light beam radiated from the light source, and NA is the numerical aperture of the objective lens.
On the other hand, if the wobbling of two neighboring tracks is out of phase to each other, the distance between the centerline of one of the recording tracks and the centerline of another recording track neighboring to the other of the recording tracks is constant and equal to twice the inherent track pitch Tp, with the recording track sandwiched between both side recording tracks deviating from the centerlines of the both side recording tracks. Thus, there persists the diffracted light deviated by λ/(2Tp·NA) from the main light beam, in a direction perpendicular to the recording track, in addition to the diffracted light deviated by λ/(Tp·NA), as shown in FIG. 24B. The result is that not only is the amplitude of the wobble signal changed, but changes in the in-spot intensity distribution different from those for the in-phase state are produced.
The changes in the amplitudes of the wobble signals produced when the wobbling of a given recording track and that of the neighboring recording track are in phase and out of phase with each other were calculated by computer simulation. The results of the calculations are shown in FIGS. 25 and 26.
Meanwhile, the graphs shown in FIGS. 25 and 26 show two-dimensional distribution, indicating changes in the wobble amplitudes, based on Z4 (defocusing) and Z9 (spherical aberration) in the Fringe-Zernike's aberration polynomial as the reference.
The “Fringe-Zernike's aberration polynomial” is now briefly explained. This polynomial is effective in representing the wavefront, because of orthogonality within the extent of a unit circle defined by a radius-azimuth polynomial of a circle, and is used often in an interferometer. If the wavefront is represented using this polynomial, for a unit circle with a radius 1,
Z1 × 1piston+Z2 × Rcos(A)tilt+Z3 × Rsin(A)tilt+Z4 × (2R2 − 1)defocus+Z5 × R2cos(2A)astigmatism in the 0° direction+Z6 × R2sin(2A)astigmatism in the 45° direction+Z7 × {(3R3 − 2R)cos(A)}coma aberration (+tilt)+Z8 × {(3R3 − 2R)sin(A)}coma aberration (+tilt)+Z9 × (6R4 − 6R2 + 1)spherical aberration (+defocus)+ . . .where R is the distance along the radial direction and A is the angle of rotation.
Meanwhile, in this computer simulation, calculations were conducted with the wavelength λ of 405 nm, the numerical aperture NA of the objective lens equal to 0.85, the track pitch Tp of the optical recording medium of 0.32 μm and with the wobble amplitude of ±10 nm. FIG. 27 depicts a graph showing changes in the amplitude of the usual push-pull signals, for reference sake.
For the in-phase case, shown in FIG. 25, the changes in the wobbling amplitude indicate two-dimensional distribution similar to the usual changes in the push-pull amplitudes shown in FIG. 27. Conversely, for the out-of-phase case, shown in FIG. 26, the changes in the wobble amplitudes indicate non-symmetrical distribution, having a gradient in an orientation two-dimensionally different from the gradients in the graphs shown in FIGS. 25 and 27.
Thus, the wobbling phase relationships of the neighboring recording tracks are periodically deviated from the in-phase state to the out-of-phase state and vice versa due to the difference between the circumference of the inner rim and that of the outer rim of the disc. As a consequence, periodic variations produced in the wobble signals may be grasped as approximately two-fold amplitude variations even in a standard state where the defocusing and spherical aberration are approximately nil. In the state where there persist defocusing and spherical aberration, there is produced a two-dimensional dissymmetry in the magnitude of the amplitude variations, due to the difference in the behavior to the defocusing and spherical aberration of the diffracted light deviated by λ/(2Tp−NA). The changes are approximately fourfold in the portions of (Def, SA)=(+, +), (−, −), as may be seen from FIGS. 25 and 27. In actuality, most of the phase states are the transition states which are neither the in-phase states nor the out-of-phase states. However, in a state deviated from the in-phase state, even to the smallest extent, similar non-symmetrical components are generated, if minor differences in the magnitude of the so generated non-symmetrical components are neglected.
If modulated components by MSK, as crucial signal components, add up to the mono-tonal wobble signal, which is the simple amplitude variations, the phase relationships of the neighboring recording tracks become more complex. That is, the MSK modulated signal portions per se represent signals having different frequencies, thus causing positive deviation in the phase of the wobbling of the neighboring recording track.
Thus, in the above-described optical head, complex variations ascribable to the deviations in the amplitude or in the phase are generated on detection of the wobble signals by the push-pull method, and hence the demodulated signals are degraded in quality, while an error rate of the preformatted signals, such as address signals or the disc information, is lowered. In addition, in a recording and/or reproducing apparatus provided with the above-described optical head for recording and/or reproducing information signals for the optical recording medium, there is produced a problem, such as delayed seek operations of the optical head or the overlooked recording errors, which might lead directly to overall deterioration in the performance of the apparatus.
On the other hand, the optical head for detecting the wobble signals is affected more seriously by the above problem if, in order to meet the demand recently raised for the high recording density, the track pitch of the optical recording medium is narrowed, because the relative wobble amplitude is then increased.
In addition, the optical head is in need of a two-dimensional margin for the spherical aberration and defocusing ascribable to an error in the thickness of the cover layer for an optical recording medium, such as the Blue-ray disc, in which it is attempted to increase the recording density by increasing the numerical aperture NA. However, in the conventional optical head in which, when the wobble signal is detected by the push-pull method, there is produced a variation in the amplitude or phase due to interference ascribable to the difference in the wobbling between the neighboring tracks, as described above, and the magnitude of the variation exhibits a two-dimensionally non-symmetrical behavior, there is produced a problem of more drastic decrease in the margin and dissymmetry. Moreover, in case of a recordable disc or a rewritable disc, signal characteristics are further deteriorated due to reflectance modulation by optical modulation recording marks.
Furthermore, in the optical head, similar problems are raised in connection with a multi-layered optical disc which is in need of a two-dimensional margin for spherical aberration and defocusing ascribable to an error in the thickness of the cover layer.